8 Then further it is to be observed that no creature can be such as its Creator. And therefore, as the divine substance or essence admits of no comparison, so neither does the Divinity. Moreover, every creature is of nothing. If therefore a spark which is so unsubstantial but yet is fire, begets of itself a creature which is of nothing, and maintains in it the essential nature of that from which it springs, (i.e. the fire of the parent spark), why could not the substance of that eternal Light which ever has been because it has in itself nothing which is not substantial, produce from itself substantial brightness? Rightly, therefore, is the Son called “only,” “unique.” For He who has been so born is “only” and “unique.” That which is unique can admit of no comparison. Nor can He who made all things be like in substance to the things which He has made. This then is Christ Jesus, the only Son of God, who is also our Lord. “Only” may be referred both to Son and to Lord. For Jesus Christ is “only” both as truly Son and as one Lord. For all other sons, though they are called sons, are so called by the grace of adoption, not by verity of nature; and if there be others who are called lords, they are called so from an authority bestowed not inherent. But Christ alone is the only Son and the only Lord, as the Apostle says, “One Lord Jesus Christ, by Whom are all things.” Therefore, after the Creed has in due order set forth the ineffable mystery of the nativity of the Son from the Father, it now descends to the dispensation which He vouchsafed to enter upon for man's salvation. And of Him whom just now it called the “only Son of God” and “our Lord,” it now says.
9. “Who Was Born by (de) The Holy Ghost of the Virgin Mary.” This nativity among men is in the way of dispensation, whereas the former nativity is of the divine substance; the one results from his condescension, the other from his essential nature. He is born by the Holy Ghost of the Virgin. Here a chaste ear and a pure mind is required. For you must understand that now a temple has been built within the secret recesses of a Virgin's womb for Him of Whom erewhile you learned that He was born ineffably of the Father. And just as in the sanctification of the Holy Ghost no thought of imperfection is to be admitted, so in the Virgin-birth no defilement is to be imagined. For this birth was a new birth given to this world, and rightly new. For He Who is the only Son in heaven is by consequence the only Son on earth, and was uniquely born, born as no other ever was or can be.
The words of the Prophets concerning Him, “A Virgin shall conceive and bring forth a Son,” are known to all, and are cited in the Gospels again and again. The Prophet Ezekiel too had predicted the miraculous manner of that birth, calling Mary figuratively “the Gate of the Lord,” the gate, namely, through which the Lord entered the world. For he says, “The gate which looks towards the East shall be closed, and shall not be opened, and no one shall pass through it, because the Lord God of Israel shall pass through it, and it shall be closed.” What could be said with such evident reference to the inviolate preservation of the Virgin's condition? That Gate of Virginity was closed; through it the Lord God of Israel entered; through it He came forth from the Virgin's womb into this world; and the Virgin-state being preserved inviolate, the gate of the Virgin remained closed for ever. Therefore the Holy Ghost is spoken of as the Creator of the Lord's flesh and of His temple.
Source: Commentary on the Apostles' Creed (New Advent)